Dye-sensitized solar cells (DSCs) have undergone continuous improvements since their introduction in 1991. DSCs have already extended into the solar energy conversion market due to their high solar-to-electric power conversion efficiencies (PCEs) and relative affordability. Despite their success so far, several key aspects of DSCs could benefit from additional improvements, including conversion of near-IR (NIR) photons and price of sensitizers. Often cited as cost-effective alternatives to metal-based sensitizers, organic sensitizers have been a key focus within DSC development. These sensitizers have tunability and have been demonstrated to productively utilize NIR photons up to 1000 nm. Organic sensitizers have seen continuous improvements leading to PCEs of >12% since the inception of DSCs.
The most common organic dye structure is the donor-π bridge-acceptor (D-π-A) configuration. Research with regard to the donor fragment has proven instrumental in increasing organic sensitizer-based DSC PCE values to >10%. High-efficiency NIR absorbing D-π-A dyes require balanced donor and acceptor strengths to avoid non-beneficial energy level perturbations. Frequently, strongly electron deficient motifs (acceptors) result in excited-state oxidation potentials that are too stabilized for electron injection into the TiO2 conduction band (CB) rendering these dyes non-functional. Accordingly, there is a need for stronger organic electron-donor materials to be matched with many of the common electron deficient DSC π-bridge-acceptor motifs. Embodiments of the present invention help meet these needs with fully-conjugated planar nitrogen-containing donor, indolizine, in model visible light absorbing systems (FIG. 1).
Nearly all organic sensitizers with >10% PCE utilize arylamine donors, either as triphenylamine, diphenylamine, or indoline. However, these donor systems are not ideal since the electron donation strength is mitigated by three main factors: (1) weak donation directionality to the dye acceptor, (2) large energy barriers to charge transfer due to the breaking of phenyl resonance stabilization energy to access the dye excited-state, and (3) non-optimal nitrogen lone pair orbital alignment with the dye π-conjugated system due to sterically induced twist angles. These mitigating factors can be substantially reduced through designing planar, fully-conjugated nitrogen-containing donor building blocks for dyes, such as indolizine-based donors. When compared computationally to triphenylamine (TPA) and diphenylamine (or indoline) based donors, dramatic improvements in nitrogen-substituent twist angles are observed. TPA shows a substantial 43° dihedral angle and indoline shows a significantly improved 23° dihedral angle. However, indolizine shows an ideal planar nitrogen-substituent bond angle. Additionally, the nitrogen lone-pair of indolizine may donate either into the 6 or 5-member ring and productively deliver the donated electron pair to the π-bridge according to valence bond theory which improves donor directionality. Given these desirable properties and a remarkably rapid, high-yielding donor synthesis (1-step in many cases), embodiments of the present invention include dyes with indolizine donors for comparison to the properties of TPA and indoline donor-based dyes with identical simple π-bridges and acceptors to clearly illustrate the effects of changing donor functionality.